1. Field of the Invention
The present invention relates to an analog switch and, more particularly to an analog switch which is suitable for use with an active matrix liquid crystal display device, and a sample and hold circuit using the same.
2. Description of the Prior Art
Driving an active matrix liquid crystal display device by sampling and holding a color TV video signal is well known in the art and may be implemented by the combination of capacitors and analog switches or CMOS switches each having the structure of an IGFET such as an MOSFET. Specifically, analog switches individually sample, for example, a red (R), a green (G) and a blue (B) component signal of a color TV video signal to store charges associated with the color component signals R, G and B in individual capacitors. Voltages associated with the charges stored in the capacitors are read out by individual buffer amplifiers to drive the liquid crystal display device.
Analog switches available for storing charges associated with signals in capacitors include an analog switch of the type having p-channel FETs and n-channel MOSFETs the source-to-drain paths of which are connected in parallel. In this type of analog switch, either one of the source electrodes and drain electrodes of the two MOSFETs is connected to a signal output to a capacitor which is adapted to store a charge while the other is connected to a video signal input. A gate or strobe signal is coupled to gate electrodes of the FETs.
Gate capacitances are parasitic on the signal output of the analog switch and the gate electrodes of the two MOSFETs. Hence, when a gate signal is applied to the gate electrodes of the FETs, there often occurs a runaround of pulse-like noises to the signal output through the gate capacitances. The pulse-like noises which reached the signal output are opposite in polarity to each other and therefore should cancel each other because the gate signals applied to the gate electrodes of the p-channel and n-channel FETs are opposite in polarity. In the prior art analog switch, however, the gate capacitance of the p-channel FET and that of the n-channel FET are not at the same level, allowing thereby noises ascribable to the gate capacitances to appear in the output signal of the analog switch.
In the prior art analog switch, the channel width differs from the p-channel MOSFET to the n-channel MOSFET and, in addition, the length over which the gate electrode overlaps impurity diffused regions which define source and drain regions differs from one of the two FETs to the other. Hence, the gate capacitances of the two FETs are different from each other. Further, when masks for forming the diffused regions and gate electrode layers are dislocated relative to each other, the gate electrodes of the p-channel and n-channel FETs may fail to have the same overlapping length. In such a condition, even if the two FETs are supplied with the same gate signal voltage, their output signals or voltages cannot be the same as each other.